Packet data transmission with optimum preamble length

ABSTRACT

In a communication network, a sending node transmits a message with a multisegment preamble and data to a receiving node. The receiving node calculates characteristics of the sending unit, channel, or transmitted signal by processing preamble segments. Once an unknown characteristic is determined, a segment of the preamble can be eliminated or reduced in length for subsequent messages, which increases efficiency of the message transmissions.

RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication Ser. No. 60/633,247 filed Dec. 3, 2004 entitled “Packet datatransmission with optimum preamble length”, incorporated herein byreference.

BACKGROUND

1. Field of Invention

The invention relates to broadband communication networks.

2. Prior Art

Data packet transmission uses a known preamble field at the beginning ofeach packet to allow a receiver to determine certain characteristics ofthe signal or channel. The preamble does not communicate any userinformation and is overhead that reduces the bandwidth efficiency of thesystem. Among the characteristics determined through processing of thepreamble are: start of packet time, carrier frequency offset, bittiming, channel response, and signal amplitude. A longer preambleenables the determination of unknown parameters with greater precision,but at the expense of reducing user data capacity.

U.S. Pat. No. 6,363,107 issued to Scott entitled “Preamble codestructure and detection method and apparatus”, incorporated herein byreference, describes a preamble structure and method of processing thepreamble for burst communication. This reference uses a combination of apulse, a dead time, and a Baker sequence for the preamble.

It is desirable to minimize the length of the preamble to increase theutilization efficiency of the communication channel.

SUMMARY OF THE INVENTION

A packet preamble has discrete segments that are each used to resolvesome uncertainty in a characteristic of the received signal andcommunication channel. Segments of the transmitted preamble areeliminated as the corresponding characteristics of the signal andchannel are determined. A specific segment of the preamble is used foreach characteristic, and therefore that respective segment of preamblecan be eliminated when the characteristic becomes known. After a segmentof the preamble is eliminated, the time interval occupied by thatpreamble segment can be re-allocated and used for transmitting userdata. Messages can be packed closer together to utilize the timeinterval saved by optimizing the preamble length.

In one embodiment, segments of the preamble are initially transmitted atone predetermined length, and then reduced to a shorter predeterminedlength. A receiving node communicates to the transmitting node toindicate that a message has been processed and an unknown characteristichas been resolved. An acknowledgement from a receiving node can be anexpress message or can be implicit in a response transmitted by thereceiving node.

In one embodiment, different preamble lengths are transmitted andprocessed differently in the receiver using either autocorrelationdetection or cross correlation detection. An initial message is sentwith a long preamble that is processed in the receiver using anauto-correlation function. Autocorrelation processing is more robust tomultipath effects and frequency offset. After determining frequencyoffset, a shorter preamble is transmitted that is processed using across-correlation function.

A segment of the preamble is used for automatic gain control (AGC)adjustment. Signal level is estimated by evaluating the first N1 bits ofthe preamble. After an estimate of signal level is made, the front-endgain is adjusted to fit the signal into the processing amplitude range.An additional N2 bits of the preamble are evaluated for a more preciseestimate. Several level estimates and correction cycles are possibleduring the AGC preamble time. After AGC converges, no AGC preamble isrequired. AGC tracking can be performed on other parts of the preambleor on the data field for fine adjustment due to environmental variationsuch as temperature.

Initial channel estimation is performed by a specific segment of thepreamble, for example using 4 orthogonal frequency division multiplex(OFDM) symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a segmented preamble according to the present invention.

FIG. 2 shows details of segments of a preamble according to the presentinvention.

FIG. 3 shows a sequence of communication between two nodes utilizing thepreamble according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a representation of message packet 100 for use with thepresent invention. Gap 105 is a time interval where no transmissionactivity occurs. A message packet comprises preamble section 110 anddata section 150.

Preamble 110 is comprised of several segments, each of which is used bythe receiver to determine a characteristic of the signal or channel.Message preamble 110 comprises a time domain preamble and a frequencydomain preamble. Each segment of the preamble is variable in length oris removed as the receiving node determines characteristics of thechannel and the transmitted signal. When the preamble is shortened, theefficiency of the communication channel is increased. By employing thistechnique, the preamble can be shortened from 6 symbols to 3 symbols.For a typical data message length of 10 symbols and a total packetlength of 16 symbols, a reduction of 3 symbols is significant.

The time domain preamble comprises short training sequence 115, longtraining sequence 120, correlator sequence 125 and is transmitted as aseries of QPSK symbols at the sample clock rate of 50 million symbolsper second (Msps), or a duration of 20 nanoseconds (nS) each.

Short training sequence 115 is used for coarse AGC setting. The signalamplitude is measured several times during this sequence and the AGCsetting is adjusted after each measurement to achieve a reasonabledegree of accuracy in the final AGC setting. The AGC setting is used forthe remaining duration of the packet. The short training sequence, whenpresent, is formed by repeating A times a predetermined short trainingpattern of 30 QPSK symbols, where A is, for example, equal to 8. Theactual value of A is determined by performance characteristics of theintegrated circuit devices used to implement the circuitry. Each30-symbol pattern is transmitted at the sample clock rate and has aduration of 0.6 microseconds (uS).

AGC is performed in the receiver by passing the received signal througha variable gain amplifier, the gain of which is controlled by a voltageor digital value. Variable gain amplifiers (VGA) are well known in theart. It is desirable that the VGA has a rapid response time in order toallow gain corrections to be made during a message. U.S. Pat. No.6,363,127 “Automatic gain control, methods and apparatus suitable foruse in OFDM receivers” to Heinonen et al., incorporated herein byreference, discloses an AGC approach for OFDM systems.

Long training sequence 120 is used for signal detection, fine frequencyoffset estimation, and timing synchronization. Long training sequence,when present, is formed by repeating B times a predetermined longtraining pattern of 64 QPSK symbols, where B is, for example, equal to 4or 8. Each 64-symbol pattern is transmitted at the sample clock rate andhas a duration of 1.28 uS.

Correlator sequence 125 is used for fine timing determination. Thecorrelator sequence, when present, is 64 or 128 symbols.

Frequency domain preamble 130 is used for channel estimation and givesthe receiver the ability to measure the multipath condition. Thisinformation can be used by the receiving node to determine optimum bitloading for the channel or bitloading can be determined by the dataportion of separate error vector magnitude (EVM) probe messages. The bitloading information is transmitted to the sending node to be used insubsequent packet transmissions. The frequency domain preamble comprisesone or more OFDM symbols transmitted with a duration of 5.12 uS each.Additional frequency domain preamble segment 135 can be used for animproved channel estimate.

Each OFDM symbol is transmitted with a cyclic prefix, also known as aguard interval, of 0 to 64 samples followed by 256 samples of data.

Data section 150 of the message packet is comprised of one or more OFDMsymbols.

Message packets are transmitted from a sending node to a receiving nodeby forming an initial message packet comprising the concatenation of amessage packet preamble comprising a plurality of segments; a data fieldcomprising a plurality of data symbols; transmitting the initial messagepacket; in a receiving node, detecting the initial message packet anddetermining at least one unknown signal characteristic of the initialmessage packet; transmitting to the sending node an indication that anunknown characteristic has been determined; in the sending node, formingan optimized message packet by removing at least one segment from thepreamble of the initial message packet; and transmitting the optimizedmessage packet.

In one embodiment, a Network Coordinator node schedules time slots inresponse to reservation requests from each sending node. When a sendingnode receives a message from the receiving node that the messagepreamble can be reduced, the sending node reserves a shorter time slotfor transmitting the message packet. The Network Coordinator node canchange the schedule of time slots, thereby reducing gap times to makeavailable a contiguous interval for use by additional messages.

A typical process would begin with the client node receiving a longpreamble packet with preamble segment 115, 120 and 125, preamble 130 andsome data symbols. The segment 115 would trigger the AGC to calculatethe amplifier gain require to bring the signal to a target referencelevel. Assuming the channel is quasi-static, this value is stored foruse by future packets. Segment 120 is used to calculate the frequencyoffset which would also be stored for use by future packets. Segment 125is then used to determine the precise start time for data symbolrecovery. Optional segment 135 can be used for improved channel estimate

With the frequency estimate and the AGC gain value estimate, subsequentpacket would only need preamble segment 125 for detection anddetermination of the precise start time of the data symbol. As frequencyestimate is expected to drift over time, adjustment can be made using afrequency-tracking loop. Likewise, energy measurement of preamblesegment 125 would be used to make small adjustment to the AGC gainvalue.

Segment 120 is designed for detection using an autocorrelation method.This method is insensitive to frequency offset and multipath. Segment125 is designed for cross-correlation detection that provides a moreprecise estimation of the packet start time. However, this method ishighly sensitive to frequency offset and requires precise knowledge ofthe multipath. Hence, it is required that frequency offset has to bepre-determined prior to using segment 125. Segment 125 further providesprotection against false detection from external sources ofinterference. It serves as a final check for false alarm. By detectingthe false alarm prior to data demodulation helps to reduce the systemoverhead as the receiver can be reinitialized and be ready to receivethe next packet without spending time analyzing the received data.

After the receiver determines the correct AGC level, or other parametersof the received message, a response message is sent to the sending unitthat indicates the AGC or other portion of the preamble can beeliminated or reduced in length. The response message can be a specificmessage or a portion of another message that carries the preamblemodification indication.

FIG. 2 shows an example of detailed parameter values for each preamblesegment. The numeric values of various parameters are dependent ondesign implementation factors, such as data rate.

FIG. 3 shows a series of message communications between two nodes in anetwork. Each node independently performs the same operations whilesending and receiving packets with the other node.

1. A method of transmitting message packets from a sending node to areceiving node comprising the steps of: in a sending node, forming aninitial message packet comprising the concatenation of: a message packetpreamble comprising a plurality of segments; and a data field comprisinga plurality of data symbols; transmitting the initial message packet; ina receiving node, detecting the initial message packet and from thedetected initial message packet, determining at least one unknown signalcharacteristic of the initial message packet; transmitting to thesending node an indication that an unknown characteristic has beendetermined; receiving in the sending node the indication that theunknown characteristic has been determined; in the sending node, inresponse to receiving the indication, forming an optimized messagepacket by removing at least one segment from the preamble of the initialmessage packet; transmitting the optimized message packet.
 2. A methodof transmitting message packets from a sending node to a receiving nodecomprising the steps of: in a sending node, forming an initial messagepacket comprising the concatenation of a time domain preamble comprisinga short training sequence, long training sequence, and correlatorsequence transmitted as a series of quadrature phase shift keyed (QPSK)symbols; frequency domain preamble comprises a plurality of orthogonalfrequency division multiplex (OFDM) symbols transmitted with a cyclicprefix; and a data section comprising a plurality of data symbolstransmitted as OFDM symbols; and transmitting the initial messagepacket; in a receiving node, detecting the initial message packet anddetermining at least one unknown signal characteristic of the initialmessage packet; transmitting to the sending node an indication that anunknown characteristic has been determined; in the sending node, inresponse to receiving the indication, forming an optimized messagepacket by removing at least one segment from the preamble of the initialmessage packet; and transmitting the optimized message packet.
 3. Amethod for transmitting message packets from a sending node comprisingthe steps of: in the sending node, forming an initial message packetcomprising the concatenation of: a message packet preamble comprising aplurality of segments; a data field comprising a plurality of datasymbols; transmitting the initial message packet using signals; inresponse to the transmission of the initial message packet, receiving anindication that a signal characteristic associated with signals used totransmit the message packet has been determined; in the sending node, inresponse to receiving the indication, forming an optimized messagepacket by removing at least one segment from the preamble of the initialmessage packet; and transmitting the optimized message packet.